Forts have no real difficulty. A fort is easy enough for a solo player. The only thing one has to worry about is the cannons as you approach the island. Those seem to be nerfed too much anyway. I soloed a fort last night and the cannons only fired one shot at my sloop. They didn't even fire as I approached the island either. A keg exploding pushed my ship away from the island into the range of a tower. That tower fired one cannonball at my ship which is what told me my ship had moved. I managed to stop fighting skeletons, run and swim back to my ship, and reposition it without a second shot ever happening.
Fighting skeletons is easy with infinite lives. You can spend as long as you want fighting and dieing until it's over.
Other crews taking control of the fort is the biggest threat. They might have a straight up ship battle with you. They might try to kill and sink you while keeping their ship at a distance. They might hide and ambush you when you unlock the vault. Forts are easy enough, it's not losing the fort/key/ treasure to other pirates that brings the challenge. The days of long fort battles between multiple crews has been missed by many.
Ship battles are much different. I've soloed many of them. Some of those I've managed to complete without sinking. They are definitely a test of your skill, your execution, your strategy and tactics, and your ability to keep cool. You have to man everything simultaneously while being aware of one or more ships that are continuously trying to put cannonballs into you. You have to know their strategy. You have to outsmart them to get the strategic advantage. You have to know your ship, knowing just where to put the wheel and sails to give you the speed and angle you want. You can't stay on the wheel because remember you need to repair and fire the cannons. Every action needs to be done quickly and confidently so you have time to do everything else. The skeletons will jump on any advantage you give them and can turn the battle around on you very fast. One mistake can ruin everything, BUT once a skeleton ship battle has started, it has to be finished. So you have an infinite number of ships to use against it. No matter how many times you sink, it will be waiting for you to come back. So again it makes the event more of a fight against time and other greedy pirates. If you throw enough ships at it, you will complete it eventually, but it can be much more time consuming than a fort. So the reward isn't worth the time and effort when compared to a fort.
With the nerfs to skeleton ships, cursed cannonballs, and the anchor no longer dropping from direct hits, the fleet battle is much easier than it was recently.